The invention relates to sewing machines in general, and, more particularly to a new and useful sewing machine having a cutting device including a bottom knife disposed in the plane of the work bearing surface and a top knife movable in a vertical direction, a drive device formed by a toggle lever transmission comprising two articulately connected connecting rods with a central joint connected with a drive element and an outer joint of one of the two connecting rods with a top knife.
In sewing machines with a cutting device for cutting the work, difficulties may arise in the trimming of work edges parallel to the sewing directions when for technical reasons the trimming device is driven, not from the drive mechanism of the sewing machine, but by its own independent drive. Since, in this case, the up and down movement of the top knife is not synchronized with the feed movement of the feed elements of the sewing machine, it may happen that the top knife is in its lowered position and cooperating with the bottom knife and touching the work precisely when the work is to be moved forward. As a result, the work piles up at the top knife and thus is pushed out of the desired feed direction.
This disadvantageous effect could be diminished by increasing the cutting frequency, because due to the greater number of cutting operations per unit time, the presence of the top knife in the position which hinders the work advance is shortened. Increased cutting frequency, however, inevitably leads to early wear of the knife edges.
West German Offenlegungsschrift No. 27 19 894 discloses a scissors, disposed crosswise to the sewing direction, which consists of a fixed lower knife and an upper knife movable up and down. The upper knife which is is connected with a toggle lever transmission formed by two connecting rods and driven by a compressed air cylinder. To carry out a cutting operation, a piston rod is moved from one end position to the other, the two connecting rods thus being swung from a first bent position through the extended position into a mirror-symmetrical second bent position. In the two bent positions of the connecting rods, the upper knife is raised, and is lowered into cutting position only in the extended position of the connecting rods. Since the piston rod moves very quickly through the position in which the connecting rods are extended and the upper knife is in cutting position, the connecting rods are swung through their extended position comparitively quickly. By this measure, the upper knife is expected to be moved, with an especially fast action, toward the end of its downward movement and at the beginning of its upward movement, so that its presence in the lowered position and hence the actual cutting operation are relatively short.
The scissors being arranged crosswise to the sewing direction may be usable for the cutting of ribbons or thread chains at the end of a workpiece and hence for a mode of operation which is interrupted by intermissions. For the trimming of work edges occurring at high frequency parallel to the sewing directions, however, it is entirely unsuitable, even if the scissors were rotated by 90.degree. and its knives would be parallel to the sewing direction. It would, in fact, be impossible with a pneumatically operating drive device, which in this case would have to have besides the air cylinder, a clock pulse generator consisting of several valves, to achieve a cutting frequency approximately corresponding to the speed of high-speed sewing machines.
The known scissors has still another fundamental disadvantage. As the connecting rods of the toggle lever transmission move from the two bent positions into the extended position, the path traveled by the joint connected with the upper knife constantly decreases in relation to the path traveled in the same time by the drive element due to the then constantly flattening angle between the connecting rods, until in the extended position of the connecting rods it is zero altogether, whereupon the knife stands still. This movement pattern is in contradiction with the effect intended by the rapid swinging of the connecting rods through their extended position, namely to obtain a shortened presence of the upper knife in the lowered position, and cancels out the intended effect at least in part.